Heretofore, in a system employing a variable resistance type controller, wherein rotation number of a sewing machine is varied by changing potential of the controller, breaking of connectors has been detected in order to prevent malfunction due to the breaking of a signal line of the connectors (and defective contact of a connector) in the controller. Detection of the breaking, however, has been imperfect. Moreover it has sometimes been seen that the variable range of the controller narrows due to the dispersion of components such as variable resistor, or that the variable potential thereof fails to rise up to a set value.